The invention generally relates to communicating with a downhole tool.
A perforating gun may be used to form tunnels in a subterranean formation for purposes of enhancing production from the formation. To accomplish this, the perforating gun typically has shaped charges that fire in response to a detonation wave propagating along a detonating cord. In this manner, the perforating gun may be lowered downhole via a tubular string (for example) until the perforating gun is at a desired depth. Some action is then taken to cause a downhole firing head to initiate the detonation wave to fire the perforating gun.
For example, one technique to cause the firing head to initiate the detonation wave involves communicating with the firing head via pressure changes that propagate through a hydrostatic column of liquid that extends from a region near the firing head to the surface of the well. In this manner, the firing head may be electrically coupled to a pressure sensor or strain gauge to detect changes in a pressure of the column of liquid near the firing head. Thus, due to this arrangement, pressure may be selectively applied to the column of liquid at the surface of the well to encode a command (a fire command, for example) for the firing head, and the resulting pressure changes that are introduced to the liquid at the surface of the well propagate downhole to the sensor. The firing head may then decode the command and take the appropriate action.
However, the above-described technique is used when the column of liquid extends to the surface of the well. The liquid may extend to the surface in overbalanced or underbalanced wells. In this manner, in overbalanced wells, the column of liquid ensures that the pressure that is exerted by the hydrostatic column of liquid near the region of perforation overcomes the pressure that is exerted by the formation once perforation occurs. The column may or may not extend to the surface of the well to establish this condition. In contrast to an overbalanced well, an underbalanced well is created to maximize the inflow of well fluid from the formation by creating, as its name implies, an underbalanced condition in which the formation pressure overcomes the pressure that is established by the column of hydrostatic liquid. The hydrostatic liquid for an underbalanced well may or may not extend to the surface of the well.
Therefore, for both underbalanced and overbalanced wells, the column of hydrostatic fluid may not extend to the surface of the well. For these cases, because the liquid does not extend to the surface of the well, the above-described technique of communicating by selectively applying pressure to the liquid at the surface of the well may not be used.
Therefore, conventionally other techniques are used to communicate commands to the firing head in an underbalanced well. For example, the firing head may respond to a bar that is dropped from the surface of the well. In this manner, the bar strikes the firing head to initiate a detonation wave on the detonating cord. It is noted that this technique may not be used in horizontal wells.
Another technique to communicate with the firing head involves the use of an expensive and complex pump system at the surface of the well to completely fill the central passageway of the string with a gas (Nitrogen, for example) to the point that the pressure is sufficient to activate the firing head. The pressurization is necessary to overcome a mechanical barrier that is associated with the firing head. For example, the pressure in the string may be increased until it reaches an absolute pressure and breaks the mechanical barrier. As an example, this mechanical barrier may be established by a shear pin that shears when the predetermined pressure differential threshold is overcome. Once the mechanical barrier is overcome, the firing head fires the perforating gun. For purposes of establishing a safety margin, the pressure differential typically must substantially exceed the nominal manufacturer-specified threshold of the mechanical barrier. Therefore, the pump system at the surface of the well must supply a large volume of gas downhole to fill the string and establish the required pressure.
The same difficulties exist in communicating with downhole tools (packers, for example) other than firing heads in an underbalanced well. Thus, there is a continuing need for an arrangement to address one or more of the problems that are stated above.
In an embodiment of the invention, a system that is usable with a subterranean well includes a downhole assembly and an apparatus. The downhole assembly is adapted to respond to a command that is encoded in a stimulus that is communicated downhole. The apparatus is adapted to change a pressure of a gas in communication with the well to generate the stimulus.
In another embodiment of the invention, a method that is usable with a subterranean well includes establishing a gas layer above a downhole assembly and selectively pressurizing the gas layer to generate a stimulus to propagate through the gas layer to the downhole assembly. The pressurization of the gas layer is controlled to encode a command for the downhole assembly in the stimulus.
In yet another embodiment of the invention, a method that is usable with a subterranean well includes receiving a stimulus downhole. The stimulus has a first pressure signature, and the first pressure signature is compared to a second pressure signature to determine an error between the first and second pressure signatures. The method includes determining whether the first pressure signature indicates a command based on the error.
Advantages and other features of the invention will become apparent from the following description, drawing and claims.